1. Field of the Invention
The present invention relates to a carbon material for electrodes which can be used for constructing electric double layer capacitors having a high electrode density and a high electrostatic capacity per unit volume.
2. Description of the Prior Art
An electric double layer capacitor (referred to as EDLC, hereinafter) is a condenser, i.e., a device for storing electricity, utilizing a double layer formed at the interface of a solid and a liquid. The structure comprises a pair of polarizing electrodes disposed on both sides of a separator, a case for containing the electrodes, an electrolytic liquid and a current collector.
As the material for the polarizing electrode, in general, active carbon having a large specific surface area is used. Examples of such active carbon include active carbons prepared from coconut shell, petroleum pitch and polyacrylonitrile. In Japanese Patent Application Laid-Open No. Heisei 9(1997)-320906, active carbons prepared from PVC and tar are described as organic materials which can be easily converted into graphite. In Japanese Patent Application Laid-Open No. Heisei 11 (1999)-293527, an electric double layer capacitor using carbon fiber derived from an optically isotropic pitch as the electrode material is described.
As for the electrolytic liquid, for example, an aqueous solution of sulfuric acid or an aqueous solution of potassium hydroxide is used as the aqueous electrolytic liquid and an electrolytic liquid of an organic solvent such as propylene carbonate in which an electrolyte such as lithium perchlorate or a quaternary ammonium salt is dissolved is used as the non-aqueous electrolytic liquid.
In preparation of the active carbon, in general, an activation process with a gas such as water vapor or carbon dioxide is conducted. To obtain an active carbon having a large specific surface area, it is necessary that the degree of activation be increased. However, the above process has a problem in that, since fine pores are formed by the reaction between the carbon material and water vapor or carbon dioxide which consumes carbon, an increase in the degree of activation results in a decrease in the yield and a decrease in the bulk density. The density of the electrode material cannot be increased, either.
The activation can also be achieved by using an alkali metal (referred to as the alkali activation, hereinafter). An active carbon having a large specific surface area and a high bulk density can be obtained at a high yield in accordance with the alkali activation since the activation proceeds in accordance with a mechanism in which carbon is not much consumed.
Since the electrostatic capacity per unit volume of EDLC can be improved by increasing the bulk density of the electrode material, it is considered that the alkali activation is the process suited for production of a carbon material for EDLC electrodes. However, the fine structure and the distribution of pores in the obtained active carbon are markedly different depending on the starting material and the condition of the heat treatment and the properties of EDLC are affected by these factors to a great degree. Therefore, it is very important that the starting material and the condition of the heat treatment are suitably selected.